The Straight Dope

Go Back   Straight Dope Message Board > Main > General Questions

Reply
 
Thread Tools Display Modes
  #1  
Old 10-08-2009, 10:28 PM
Campion Campion is offline
Guest
 
Join Date: Mar 2005
How does deforestation decrease rainfall?

This NASA article posits that the "Mayans did it to themselves." Among other things, the article suggests that deforestation -- required to clear land to plant crops to support a growing population -- led to lower rainfall, which of course led to fewer crops, which of course required more deforestation, lather, rinse, repeat, until eventually the Mayans died out.

So how does deforestation affect rainfall?
Reply With Quote
Advertisements  
  #2  
Old 10-08-2009, 11:04 PM
Telperion Telperion is offline
Guest
 
Join Date: Jul 2005
I guess it's basically the same thing that happened in North Africa. When part of the plantlife is removed there is nothing left holding moisture on the ground, so the rainwater runs down in deep aquifers and there is very little evaporation in the area, which in turn leads to less rain and even less plantlife. A century or so later what you have is practically a desert since very little can grow in dry earth.
Reply With Quote
  #3  
Old 10-08-2009, 11:22 PM
groo groo is offline
Charter Member
 
Join Date: Aug 2001
Location: Los Angeles
Posts: 1,002
Deforestation leaves the ground vulnerable to rapid erosion -- the water is no longer held in the roots and the plants/trees themselves, so when it rains, the water rapidly washes away, resulting in the overall moisture leaving the area and going far away, eventually to the ocean. The main point is that it doesn't have time to soak into the ground and stick around locally, so it won't evaporate and create clouds locally, and so it will no longer rain as often ... it's just no longer in the area.

Oops! Looks like Telperion explained it while I was typing.

Last edited by groo; 10-08-2009 at 11:23 PM..
Reply With Quote
  #4  
Old 10-09-2009, 08:47 AM
Quercus Quercus is offline
Guest
 
Join Date: Dec 2000
It's not that trees 'hold' moisture, but that they put it into the air. For leaves to work, the have to be both moist and exposed to the atmosphere, so lots of water evaporates from leaves. I can't look it up right now, but something like a a third to a quarter of the rainfall on a typical temperate forest is pulled out of the ground by trees and evaporates. I imagine in a more tropical area that percentage is even higher. Crops are going to have less leaf area that trees, so less evaporation. With less water being put into the air, there's less rain downwind.

To some degree, crops are going to have more surface run-off (and more erosion), but as far as affecting local rainfall, the leaf evaporation is going to be the big effect.


Though my shallow understanding was that soils in tropical forests are extremely vulnerable to erosion and nutrient depletion, so that their productivity for crops declines dramatically after a few years. To my non-expert perspectice, this seems more likely to be a problem than shifting rainfall patterns due to land cover changes. [And looking at the article, they certainly mention this, too]
Reply With Quote
Reply



Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 01:49 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.

Send questions for Cecil Adams to: cecil@chicagoreader.com

Send comments about this website to: webmaster@straightdope.com

Terms of Use / Privacy Policy

Advertise on the Straight Dope!
(Your direct line to thousands of the smartest, hippest people on the planet, plus a few total dipsticks.)

Publishers - interested in subscribing to the Straight Dope?
Write to: sdsubscriptions@chicagoreader.com.

Copyright 2013 Sun-Times Media, LLC.